A reversibly thermochromic ultraviolet ray-hardening ink composition containing a microencapsulated pigment in which a reversibly thermochromic composition comprising (A) an electron-donating compound, (B) an electron-accepting compound and (C) a medium which controls color reaction generation temperature of the former two compounds is contained has been disclosed (U.S. Pat. No. 5,500,040).
In general, light resistance of an electron-donating compound (A) in a reversibly thermochromic composition is not strong, so that an ultraviolet ray-hardening ink composition prepared using the same also has the same problem.
Accordingly, in the conventional ink, the ultraviolet ray absorbent contained in microcapsules and absorption wavelength of the photo-polymerizable composition contained in the vehicle are specified in order to improve light resistance of the reversibly thermochromic composition and to maintain the ultraviolet ray-hardening property. That is, by the use of a photo-polymerization initiator having a maximum absorption wavelength at more higher wavelength side of the ultraviolet ray absorbent having the effect to improve light resistance of the reversibly thermochromic composition, the ultraviolet ray absorbent does not hinder hardening generated by ultraviolet ray irradiation and, what is more, light resistance of the reversibly thermochromic composition is improved by the ultraviolet ray absorbent.
However, though a light resistance improving effect can be obtained by the conventional ink, joint use of the ultraviolet ray absorbent is apt to cause an inconvenience of spoiling color changing sensitivity which is one of the important color changing functions of the reversibly thermochromic composition, so that it has a disadvantage in that sharp coloring and discoloring or change of a color into another color having different tone cannot be shown by changes in temperature.
In addition, though formulation of the ultraviolet ray absorbent shows the effect to improve light resistance under discolored condition, it does not show sufficient effect to improve light resistance under colored condition.